1. Field of the Invention
The present invention relates to a control apparatus suitable for use in an induction motor, which permits rotational-speed and rotational-position controls.
2. Description of the Prior Art
FIG. 1 is a block diagram showing an inverter-drive apparatus of a vector control type, which is used as a control apparatus for an induction motor. In the drawing, designated at numeral 1 is a three-phase alternating current power supply. Numeral 2 indicates a converter in which diodes or the like are employed, said converter serving to rectify an alternating current delivered from the three-phase alternating current power supply. Numeral 3 denotes a filter capacitor for smoothing out a voltage rectified by the converter 2. Numeral 4 denotes inverter composed of transistors or the like, and which in turn serves to convert a d.c. voltage smoothed by the filter capacitor 3 into a three-phase alternating voltage. Numeral 5 denotes an induction motor (hereinafter called "Electric Motor") which is to be driven by a three-phase alternating voltage outputted from the inverter 4. The electric motor 5 is coupled to a principal axis of an unillustrated machine tool.
Designated at numeral 6 is a velocity detector which is mounted on the electric motor 5 and serves to output a signal corresponding to the rotational speed of the electric motor 5. Numeral 7 indicates a high-resolution position detector which is attached to the electric motor 5 and serves to output a signal corresponding to the rotational position of the electric motor 5.
Numeral 8 indicates a numerically controlled apparatus for outputting a velocity command .omega..sup.*.sub.r or position command .theta..sup.*.sub.r. Numeral 9 denotes a velocity-command production circuit which serves to transfer a velocity command delivered from the numerically controlled apparatus 8 as is, during operation of the motor in a rotational-speed control mode. In addition, the velocity-command production circuit also serves to compare a position command .theta..sup.*.sub.r delivered from the numerically controlled apparatus 8 during operation of the motor in rotational-position control mode with a position detection signal .theta..sub.r from the position detector 7 so as to obtain a deviation signal, thereby outputting a velocity command .omega..sup.*.sub.r computed based on the deviation signal. Designated at numeral 10 is a vector-control arithmetic circuit which serves to subject the velocity command .omega..sup.*.sub.r delivered from the velocity-command production circuit 9 and a velocity detection signal .omega..sub.r from the velocity detector 6 to a vector-control computation, to thereby output the amplitude .vertline.I.sub.1 .vertline., the angular velocity .omega..sub.0 and a phase angle .DELTA..theta. of the primary current, which are to be applied to the electric motor 5. Numeral 11 indicates a primary-current reference generation circuit for producing a U-phase primary current command i.sup.*.sub.US and a V-phase primary current command i.sup.*.sub.VS based on the amplitude .vertline.I.sub.1 .vertline., the angular velocity .omega..sub.0 and the phase angle .DELTA..theta. of the primary current, which are delivered from the vector-control arithmetic circuit 10. Designated at numeral 12 is a current control circuit which serves to compare the primary current commands i.sup.*.sub.US, i.sup.*.sub.VS fed from the primary-current reference generation circuit 11 with feedback signals i.sub.US, i.sub.VS of the primary current, which flow through the electric motor 5 so as to obtain a deviation signal, thereby outputting a signal for controlling an output current of the inverter 4, based on the deviation signal. Incidentally, the control circuit portion for the inverter 4 is composed of the velocity-command production circuit 9, the vector-control arithmetic circuit 10, the primary-current reference generation circuit 11 and the current control circuit 12.
FIG. 2 is a block diagram showing, in detail, the velocity-command production circuit 9, the vector-control arithmetic circuit 10 and the current control circuit 12 depicted in FIG. 1. In the drawing, designated at numeral 13 is a position loop gain circuit for inputting a deviation signal equal to the difference between a position command signal .theta..sup.*.sub.r and a position detection signal .theta..sub.r so as to multiply the same by a position loop gain K.sub.PP, thereby outputting a velocity command .omega..sup.*.sub.r. Numeral 14Z indicates a PI (Proportion/Integral) control circuit as a velocity loop gain circuit, which serves to subject a deviation signal corresponding to the difference between the velocity command .omega..sup.*.sub.r and the velocity detection signal .omega..sub.r to a proportional/integral control computation so as to output the results. Designated at numeral 15 is a limiter circuit which serves to limit an output from the PI control circuit 14Z to a constant saturated value i.sup.*.sub.qs max so as to be a torque-component current command i.sup.*.sub.qs. Numeral 16 indicates a weakening variable-flux generation signal output circuit which is operative to output a signal .phi..sub.2 corresponding to the secondary flux in conformity with an output i.sup.*.sub.qs of the limiter circuit 15 based on the velocity detection signal .omega..sub.r. Designated at numeral 17 is a first-order lag element for outputting the secondary flux command .phi..sup.*.sub.2 based on the secondary flux generation signal .phi..sub.2, and numeral 18 indicates a mutual reactance pattern generation circuit for producing the mutual reactance M for the electric motor from the secondary flux command .phi..sup.*.sub.2. Numeral 19 indicates an excitation-component current arithmetic circuit for outputting an excitation-component current command i.sup.*.sub.ds based on the secondary flux generation signal .phi..sub.2 and the mutual reactance M, and numeral 20 denotes an amplitude arithmetic circuit for computing the amplitude .vertline.I.sub.1 .vertline. of the primary current based on i.sup.*.sub.qs and i.sup.*.sub.ds. Designated at numeral 21 is a phase-angle arithmetic circuit for performing an arithmetical operation of the phase angle .DELTA..theta. of the primary current based on i.sup.*.sub.qs and i.sup.*.sub.ds, and numeral 22 indicates a slip angle-frequency arithmetic circuit for computing a slip angle frequency .omega..sub.s based on i.sup.*.sub.qs and i.sup.*.sub.ds. Numerals 23 and 24 indicate current loop gain circuits for multiplying the difference between the primary current command i.sup.*.sub.US and the primary current detection signal i.sub.US and the difference between the primary current command i.sup.*.sub.VS and the primary current detection signal i.sub.VS by a current loop gain K.sub.PI, to thereby obtain voltage commands V.sup.*.sub.VS and V.sup.*.sub.VS. Designated at numeral 25 is a pulse width modulation circuit (hereinafter called "PWM" circuit) for generating PWM signals for determining the ON and OFF times of each of the transistors of inverter 4 based on V.sup.*.sub.US and V.sup.*.sub.VS. In addition, numeral 26 is a control-mode change-over switch for selecting either of the rotational-speed control mode and the rotational-position control mode.
A description will next be made of the operation of the control apparatus. From a known vector control theory, it follows that ##EQU1## where T.sub.M =desired generation torque of induction motor
P.sub.m =number of pole pairs PA1 R.sub.2 =secondary resistor PA1 L.sub.2 =secondary reactance PA1 i.sub.qs =torque-component current PA1 i.sub.ds =excitation-component current PA1 S=differential operator
Meanwhile, in vector control, a deviation signal corresponding to the difference between the velocity command signal .omega..sup.*.sub.r and the velocity detection signal .omega..sub.r is amplified by the PI control circuit 14Z and a constant limitation is imposed on the amplified signal by the limiter circuit 15, to thereby output the torque-component current command i.sup.*.sub.qs. In accordance with the formula (2), the excitation-component current arithmetic circuit 19 performs a first-order lag operation of a constant L.sub.2 /R.sub.2 on signals corresponding to the secondary flux .phi..sub.2, which is obtained from the weakening variable-flux generation signal output circuit 16 based on the velocity detection signal .omega..sub.r and the torque-component current command i.sup.*.sub.qs, and an operation of multiplying the result by the mutual reactance M obtained from the mutual reactance pattern generation circuit 18, thereby obtaining an excitation-component current command i.sup.*.sub.ds. On the other hand, the slip angle frequency .omega..sub.s can be obtained from the formula (3) by, at the slip angle-frequency arithmetic circuit 22, dividing the torque-component current command i.sup.*.sub.qs by the secondary flux command .phi..sup.*.sub.2 and then multiplying the result by the coefficient (R.sub.2 /L.sub.2).multidot.M.
The amplitude .vertline.I.sub.1 .vertline., the angle frequency .omega..sub.0 and the phase angle .DELTA..theta. of the primary current command can be determined by the following equations: ##EQU2##
Accordingly, the arithmetical operation according to the equation (4) is performed in the amplitude arithmetic circuit 20 and that according to the equation (6) is carried out in the phase-angle arithmetic circuit 21.
In the vector control apparatus which has been constructed as described above, the switch 26 is set to the "A" side in FIG. 2 in the normal rotational-speed control of the electric motor 5, i.e., the principal-axis operation mode such that a velocity loop for controlling the rotational speed of the electric motor 5 is formed. In addition, the switch 26 is set to the "B" side in the rotational-position control mode, i.e., the C-axis mode such that a position loop for controlling the rotational position of the electric motor 5 is formed. The response at the time of the C-axis mode is determined by the position loop gain K.sub.PP which is to be set in the position loop gain circuit 13 in the velocity-command production circuit 9, and velocity loop proportional and integral gains K.sub.PV and K.sub.IV which are to be set in the PI control circuit 14Z in the vector-control arithmetic circuit 10. The position loop gain K.sub.PP is normally set to a value of 30 sec.sup.-1 or so. The velocity-loop proportional and integral gains K.sub.PV and K.sub.IV are set to a larger value wherever practicable, to the extent that the velocity control system is not rendered unstable, without imposing particular limitations on the principal-axis mode and the C-axis mode, to thereby improve the response.
In the control apparatus for use in the conventional induction motor, which has been constructed as described above, the velocity loop gains K.sub.PV and K.sub.IV are fixed values. Therefore, the velocity response of the vector-control arithmetic circuit 10 at the time of the rotational-speed control mode, i.e., the principal-axis mode is the same as that of the vector-control arithmetic circuit 10 at the time of the rotational-position control mode, i.e., the C-axis mode. It is also necessary to determine the gain in such a way that it is rendered stable at full rotational regions of the motor. Thus, the gain has generally been set to a lower value. In addition, the secondary flux .phi..sub.2 of the electric motor 5 is set to about 1/2 of the rating upon non-load and the excitation-component current i.sup.*.sub.ds is set such that the secondary flux becomes gradually greater up to 100% rated flux as the load increases. Accordingly, the velocity response of the vector-control arithmetic circuit 10 at the time of the principal-axis mode is the same as that of the vector-control arithmetic circuit at the time of the C-axis mode.
However, where a machine tool, e.g. an end mill is employed for a C-axis cutting use, an unstable external force is applied to the electric motor 5 owing to blade contacts of cutting tools. If the velocity response of the electric motor 5 is not precise, a change in velocity takes place due to the unstable external force on the motor thereby causing large variations in the difference between an intended command position and an actual position. As a consequence, the accuracy of the C-axis cutting is deteriorated. If the cutting of objects to be cut by the cutting tools is made small with a view toward reducing the variations in the positional errors, the limit capability of the cutting is deteriorated, thereby resulting in reduced applicability of the machinery.